This invention relates to a soft mute circuit; that is, a circuit for masking transients in an audio electronic device. As used herein, a “transient” is an abrupt change in the operation of a circuit or a spurious signal caused by such abrupt change.
Anyone who has ever put on earphones before plugging the earphones into an operating radio, stereo, or cellular telephone knows well the sound of transients. Other transients occur during the operation of audio electronic devices. In a device such as a telephone or a hearing aid, the transients can be particularly annoying. Such transients arise from switching circuitry within the device as the device changes state. Telephone systems, for example, have at least two channels and a plurality of filters in each channel. The various combinations of channels and filters are switch selected and the changes can be heard easily, to the annoyance of the user.
In the prior art, such transients were generally handled by filtering or by carefully matching voltage levels. U.S. Pat. No. 4,983,927 (Torazzina) discloses a bias circuit that causes a power amplifier to go through “mute” and “standby” states when the amplifier changes from normal operation to “cut-off” for blocking transients.
Unlike the Torazzina patent, it is desired to selectively mute signals from a plurality of sources. It is also desired to control the depth and duration of the mute better.
In view of the foregoing, it is therefore an object of the invention to provide an improved mute circuit for unobtrusively masking transients in an audio device.
Another object of the invention is to provide a mute circuit that can operate on several signals in any combination.
A further object of the invention is to provide a mute circuit wherein the depth and duration of the mute are adjustable.
Another object of the invention is to provide a soft mute for a telephone.